There are two types of inks used for ink-jet printing: oily inks and aqueous inks. Since the oily inks have problems due to unpleasant odors and toxicity, aqueous inks are widely used currently.
However, conventional aqueous inks have drawbacks in water proofing and fade-resistance qualities, because it uses water soluble dyes as the coloring agent. Moreover, since the dye is dissolved to its molecular level, the quality of the printing is extremely deteriorated because beard-like bleeding called "feathering" occurs, when printing is performed using ordinary paper which is normally used in offices, for example, for copiers.
Conventionally, so called "aqueous pigment inks" are proposed. Examples of these inks include an ink and a polymer latex which contain carbon black or an organic pigment dispersed in a water-soluble resin used for both the purposes of a binder and of a dispersing medium, and a dispersion-type ink composed of microcapsules involving a colorant agent which is dispersed in a resin dispersion medium.
As an aqueous pigment ink for a ink-jet printer, it is necessary to use fine colorant particles dispersed as finely as possible in an aqueous medium. An example of the aqueous ink of a resin solution type is shown in Japanese Patent No. 2512861, which discloses a method of preparation of an aqueous ink with improved characteristics, comprising the steps of: (1) supplying a pigment and a polymer dispersion agent into a twin roll milling device, (2) obtaining a dispersion product of the pigment and the polymer dispersion agent by milling, and (3) dispersing the dispersion product in an aqueous carrier medium. Japanese Patent Application, First Publication, No. Hei 3-153775 discloses an aqueous ink composition for ink-jet printing comprising (a) a solid mixture of a pigment and a polyacrylate resin containing a carboxyl group, (b) an organic solvent which can be diluted by water, (c) a wetting agent, and (d) water. Although these techniques are effective in pulverizing the pigment into fine particles, the printing operation is often interrupted by an extraordinary emission of ink due to the increase of the viscosity of the ink caused by evaporation of water in the ink, and in the worst cases, the jet nozzle is clogged, and it is observed that the water proof quality of the printed matter is poor.
In general, the resin-dispersion-type aqueous ink is advantageous in the low susceptibility for the viscosity increase by water evaporation. Practically, Japanese Patent Application, First Publication, No. Sho 58-45272 discloses an ink composition including an urethane polymer latex containing dyes, and Japanese Patent Application, First Publication, No. Sho 62-95366 proposes an ink containing a dye packed in polymer particles which are produced by dissolving a polymer and a solvent dye in a water insoluble organic solvent, mixing with a water solution containing a surface active agent and emulsifying the mixture, and after evaporating the solvent, obtaining an ink containing a dye involved in polymer particles. Japanese Patent Application, First Publication, No. Sho 62-254833 proposes a method of making an aqueous dispersion of the colorant by decreasing the surface tension at an interface between the organic solvent and water to less than 10 dyne at the time of the encapsulation process. Japanese Patent Application, First Publication, No. Hei 1-170672 proposes a recording liquid containing the microencapsulated dye. However, the above colorant dispersions are not necessarily sufficient in the dispersion stability and are also not necessarily satisfactory in the emission property of the ink-jet because of high foaming properties due to the effect of the surface active agent used in the encapsulation process.
Japanese Patent Application, First Publication, No. Hei 3-240586 proposes an image forming material, wherein the surface of dispersed particles in the dispersion medium is covered with a resin which swells by the dispersing medium. However, since the phase transformation of sol-gel easily occurs in a temperature range near room temperature, and since the dispersed particles are not stable in the dispersion medium, the image forming material often causes extraordinary emissions.
Japanese Patent Application, First Publication, No. Hei 5-247370 discloses a coloring composition for image recording comprising a pigment and a resin, wherein the pigment is coated with a resin which is substantially insoluble to the dispersion medium and which covers the pigment in a form of a cured polymer film having polar groups. However, since the above composition lacks a self-dispersion ability and lacks an adhering ability on a recording paper, it is necessary to incorporate resins as a dispersing agent and as a fixing agent, which hinder improvement of drawbacks such as low stability in emission of the ink and low water proof quality of the printed image, even though the dispersion stability is better than the ink containing the pigment without the coating of the cured resin.
Furthermore, Japanese Patent Application, First Publication, No. Hei 2-255875 discloses an aqueous ink for an ink-jet printer comprising a pigment, a water soluble resin, water soluble organic solvent, and water, wherein "resin adsorbed pigment particles", in which the water soluble resin is fixed on the pigment particles by a weak binding force, are dispersed in the aqueous medium, and in the aqueous medium, a limited amount of the water soluble resin is dissolved to less than 2% by weight, without being adsorbed on the pigment particles.
However, since the above method fails to provide a sufficient binding force between the pigment particles and the resin, a higher dispersion stability can be anticipated by the use of such "resin adsorbed pigment particles".
In addition, in the case when such "resin adsorbed pigment particles" are used, it will not be expected for an ink to be provided with both good emission property and good adhering property to the recording sheet, even if the quantity of the aqueous resin dissolved in the ink without being adsorbed by the pigment is restricted to less than 2% by weight.
The objects of the present invention are to provide a color microencapsuled-type aqueous jet ink which contains fine particles and is superior in dispersion stability, ink emission stability, and in adherence of microcapsules on the recording medium.